Optical communication is a technique that could play a central role in future broadband communications. For widespread use of the optical communication, the optical components used in optical communication systems are required to be higher in performance, smaller in size, and lower in price. Optical communication devices using photonic crystals are one of the leading candidates for the next-generation optical communication components that satisfy the aforementioned requirements.
A photonic crystal is a dielectric object having an artificial cyclic structure. Usually, the cyclic structure is created by providing the dielectric body with a cyclic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the body. Within the crystal, the cyclic structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap (PBG)”. The energy region (or wavelength band) at which the PBG is created depends on the refractive index of the dielectric body and the cycle distance of the cyclic structure.
Introducing an appropriate defect into the photonic crystal creates a specific energy level within the PBG (“defect level”), and only a ray of light having a wavelength corresponding to the defect level is allowed to be present in the vicinity of the defect. This means that a photonic crystal having such a defect can function as an optical resonator that resonates with light having a specific wavelength. Furthermore, forming a linear defect enables the crystal to be used as a waveguide.
Under the condition that the body is a silicon plate and the modified refractive index area is made of air (or a hole), the cyclic distance of the crystal should be 1 μm or smaller for the near infrared light commonly used in modern optical communications, whose wavelength is from 1.25 to 1.65 μm. Manufacturing such small structures requires an accuracy level of the nanometer order. Recent improvements in manufacturing machines have enabled the nanometer-scale working process to be applied to some types of photonic crystals for optical communications, which have already been put into practical use. An example is a photonic crystal fiber for polarization dispersion compensation. Furthermore, recent efforts have had a practical goal of developing optical multiplexers/demultiplexers and other devices that can be used in wavelength division multiplexing.
Patent Document 1 discloses a two-dimensional photonic crystal having a body (or slab) provided with a cyclic arrangement of modified refractive index areas, in which a linear defect of the cyclic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a ray of light whose wavelength equals the resonance frequency of the resonator from rays of light having various wavelengths and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Many two-dimensional photonic crystals are designed so that the PBG becomes effective for either a TE-polarized light, in which the electric field oscillates in the direction parallel to the body, or a TM-polarized light, in which the magnetic field oscillates in the direction parallel to the body. For example, if the cyclic structure has a triangular lattice pattern and each modified refractive index area is circular (or cylindrical), the PBG is created for only the TE-polarized light. A waveguide or resonator using such a two-dimensional photonic crystal is almost free from loss as far as the TE-polarized light is used. However, since it has no PBG created for the TM-polarized light, the crystal body allows the TM-polarized light to freely propagate through it. Therefore, if a ray of light containing both kinds of polarized light is introduced into the waveguide or resonator consisting of a two-dimensional crystal, one of the two polarized lights leaks from the waveguide or resonator into the body, which deteriorates the light-propagating efficiency.
Taking the above problem into account, studies have been conducted on a new design of two-dimensional photonic crystal having a PBG for each of the TE-polarized light and the TM-polarized light in which the two PBGs have a common band. This common band is called the “absolute photonic band gap (absolute PBG)” hereinafter. For example, FIG. 1(a) is a plan view of a two-dimensional photonic crystal disclosed in Non-Patent Document 1, which has an absolute PBG created by cyclically arranging triangular (or triangle-pole-shaped) holes 12 in a triangular lattice pattern in the slab 11. Within this two-dimensional photonic crystal, neither the TE-polarized light nor the TM-polarized light can leak from the waveguide, resonator or other device into the body as long as the wavelength of the light is within the absolute PBG. Therefore, the efficiency is maintained.
In the two-dimensional photonic crystal disclosed in Non-Patent Document 1, the absolute PBG can be widened by increasing the filling factor (FF), an area fraction of the holes (i.e. modified refractive index areas) within one lattice unit with respect to the area of the lattice unit. Thus, one can broaden the wavelength band available.
Practically, however, the construction in Non-Patent Document 1 does not allow the FF value to be equal to or larger than 0.5 because the neighboring holes 12 are in contact with each other when the FF value is 0.5, as shown in FIG. 1(b). Moreover, even if the value is smaller than 0.5, a larger FF value makes the connecting portion of the body thinner at each corner of the triangle and thereby weakens the slab 11. Therefore, the FF value practically needs to be equal to or smaller than 0.45. Thus, the construction in Non-Patent Document 1 has limitations relating to the setting range of the absolute PBG and the breadth of the wavelength band available, which depends on the absolute PBG.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2001-272555 ([0023]-[0027], [0032], FIGS. 1, and 5-6)
[Non-Patent Document 1] Hitoshi KITAGAWA et al. “Nijigen Fotonikku Kesshou Surabu Ni Okeru Kanzen Fotonikku Bando Gyappu (“Absolute photonic bandgap in two-dimensional photonic crystal slabs)”, Preprints of the 50th Joint Symposia on Applied Physics, Japan Society of Applied Physics, March 2003, p. 1129